Patent Document 1: International Publication No. WO 2004/079806
To advance the development of next-generation DRAMs, a challenge is to reliably provide the capacity of capacitors while the cell area is decreasing in association with finer design rules. In DRAMs up to 16-Mbit ones, mutilevel structures such as a stack type, a trench type, and a fin type are adopted for the cell structures of capacitors. However, in order to fabricate 256-Mbit DRAMs or above with the use of these mutilevel structure capacitors, problems are an increase in the number of process steps caused by complicated processes and a reduction in yields because of increases in step height. Therefore, in recent years, such studies are proceeding that thin films using high dielectric constant materials such as Ta2O5, Y2O3, and HfO2 are used for dielectric films of capacitors. Moreover, as materials having a dielectric constant higher than that of these oxide materials and having expectation for application to DRAMs, (baxSr1-x)TiO3, Pb(ZryTi1-y) O3, and (PbaL1-a)(ZrbTi1-b) O3 are thought as promising ones. In addition, Bi-layer ferroelectric materials having a crystal structure very similar to that of superconducting materials are also hopeful, and in recent years, attention is particularly focused on SrBi2TaO9 called a Yl material because of its excellent drive at low voltage and fatigue characteristics. Generally, the formation of an SrBi2TaO9 ferroelectric thin film is conducted according to practical, promising MOCVD (Metal Organic Chemical Vapor Deposition) methods.
Raw materials for ferroelectric thin films are generally three types of organometallic complexes, Sr(DPM)2, Bi(C6H5)3, and Ta(OC2H5)5, and each of these complexes is dissolved in a THF (tetrahydrofuran) solvent for use as a liquid solution. In addition, DPM is an abbreviation of dipivaloylmethane.
Their material properties are shown in Table 1.
TABLE 1Boiling point(° C.)/pressure (mmHg)melting point (° C.)Sr(DPM)2242/1478Bi(C6H5)3270 to 280/1201Ta(OC2H5)5146/0.1522THF67−109
A system used for the MOCVD method is configured of a supplying unit that supplies an SrBi2TaO9 thin film raw material and an oxidizing agent to a reaction unit, the reaction unit that causes vapor phase reaction and surface reaction on the SrBi2TaO9 thin film raw material for film deposition, and a collecting unit that collects products generated in the reaction unit. Then, the supplying unit is provided with an evaporating apparatus for evaporating a thin film raw material.
As an evaporating apparatus before, a metal filter evaporating apparatus is known in which a raw material solution heated at a predetermined temperature is dropped onto a metal filter used for the purpose of increasing the contact area between an ambient gas and an SrBi2TaO9 ferroelectric thin film raw material solution, thereby conducting evaporation. However, in this technique, there is a problem that the metal filter is clogged for several times of evaporation and the filter cannot be used for a long time.
In addition, when the raw material solution is a mixed solution of a plurality of organometallic complexes, a mixed solution of Sr(DPM)2/THF, Bi(C6H5)3/THF, and Ta(OC2H5)5/THF, for example, and this mixed solution is evaporated by heating, the solvent having the highest vapor pressure (in this case, THF) is first evaporated and the organometallic complexes are deposited and attached on the heating surface, and on this account, such a problem also arises that a raw material cannot be stably supplied to the reaction unit.
As a technique for solving these problems, an evaporating apparatus disclosed in Patent Document 1 is known. This evaporating apparatus is configured of a dispersing unit that has a gas passage provided with a cooling means, the dispersing unit bringing a pressurized carrier gas and a raw material solution into the gas passage for delivering the carrier gas containing the raw material solution to an evaporating unit, and the evaporating unit that heats and evaporates the carrier gas containing the raw material solution delivered from the dispersing unit.
FIG. 8 is a cross section depicting a dispersing unit of an MOCVD evaporating apparatus according to the background technique disclosed in Patent Document 1. An evaporating apparatus 201 according to the background technique is an evaporating apparatus that brings in a carrier gas from one ends of gas passages 206 and 207, and delivers a carrier gas containing a raw material solution from an outlet port 208, which is the other end of the gas passages 206 and 207, to an evaporating unit for evaporation. Mass flow controllers (MFCs) 209 and 210 are provided on one ends of the gas passages 206 and 207, respectively, and manometers 202 and 203 are provided, which are means for detecting pressure inside the gas passages 206 and 207. Pressure inside the gas passage is controlled by the MFC, and pressure inside the gas passage is detected at the same time, whereby clogging in the gas passage can be suppressed and the timing can be informed in advance that dumps are needed to clean.
As shown in FIG. 8, the dispersing unit of the evaporating apparatus according to the background technique is configured of the gas passages 206 and 207 that bring in a carrier gas from the upper direction and solution passages 211 and 212 that bring in a raw material solution from the lateral direction. The raw material solution is issued into the carrier gas in the midway of the gas passage for atomization, the mist is mixed with the carrier gas, the carrier gas is brought together with one in the other passage in the upper part of the outlet port 208, and a plurality of the carrier gases mixed with different raw material solutions is issued into the evaporating unit and heated in the evaporating unit, whereby an MOCVD film is deposited in a depositing unit.
As shown in FIG. 8, the portion in which the raw material solutions in the gas passages are mixed and gases are issued from the outlet port 208 is called a center rod head, in which a rod is provided in the center of the pipe for concentrating and issuing gases, and the gas pipe is tapered at an angle of about 20 degrees. On this account, it is difficult to conduct the position adjustment and centering of the gas passages in the dispersing unit. In addition, the portion at which the solution passage is mounted on the gas passage has a tapered cylindrical surface, and individual evaporating apparatuses are different from each other, and thus it is not easy to conduct accurate control of MOCVD film deposition processing.
In addition, as described above, because the boiling point of the solvent of the raw material solution is lower than the boiling point of the organometallic raw material, it is necessary to cool the dispersing unit in order to prevent clogging caused by the deposit of organic metals. However, in the evaporating apparatus according to the background technique, because the gas passages 206 and 207 and the solution passages 211 and 212 are separate pipes, it is difficult to uniformly cool all the pipes. Therefore, there is still a problem that it is not easy to conduct accurate control of MOCVD film deposition processing. Moreover, it is difficult to increase the number of gas passages, three or four gas passages can be provided at best, and the evaporating apparatus is not ready for the formation of sophisticated MOCVD films using a wide variety of raw materials. In addition, because the evaporating apparatus has the structure in which the gas passages are arranged in the upper part, such a problem also arises that the height of the evaporating apparatus becomes higher to increase the apparatus size.